U.S. officials say the United States plans to announce new sanctions targeting Syrian President Bashar al-Assad's regime and its supporters.

Senior officials traveling with U.S. Secretary of State Hillary Clinton on her Africa tour said the fresh sanctions aimed at speeding Assad's ouster were imminent.

The sanctions are expected to complement existing penalties, which have targeted Assad's inner circle, along with Iranian individuals and entities that have been providing support to the Syrian regime.

They come as Clinton prepares for discussions in Istanbul on August 11 that will focus on transition in a post-Assad Syria.

Meanwhile, the United Nations says Syrian civilians continue fleeing the fighting in Syria, bringing the total to nearly 150,000 refugees registered in four neighboring countries since the conflict began.

Adrian Edwards, spokesman of the UN High Commissioner for Refugees, told a news briefing in Geneva that the total included the more than 50,000 refugees recorded in Turkey, where over 6,000 Syrians arrived this week alone.

Edwards said that almost 46,000 Syrian refugees were registered in Jordan, up to 37,000 in Lebanon, and some 13,500 in Iraq -- which has also seen the return of more than 23,000 Iraqis from Syria since July 18.